On Collide add progress help

Hi Everyone! I have a problem with progress achievement system. using System.Linq; using UnityEngine; using System.Collections;

 [System.Serializable]
 public class Achievement
 {
     public string Name;
     public string Description;
     public Texture2D IconIncomplete;
     public Texture2D IconComplete;
     public int RewardPoints;
     public float TargetProgress;
     public bool Secret;
 
     [HideInInspector]
     public bool Earned = false;
     private float currentProgress = 0.0f;
 
     public bool AddProgress(float progress)
     {
         if (Earned)
         {
             return false;
         }
 
         currentProgress += progress;
         if (currentProgress >= TargetProgress)
         {
             Earned = true;
             return true;
         }
 
         return false;
     }
 
     public bool SetProgress(float progress)
     {
         if (Earned)
         {
             return false;
         }
 
         currentProgress = progress;
         if (progress >= TargetProgress)
         {
             Earned = true;
             return true;
         }
 
         return false;
     }
 
     public void OnGUI(Rect position, GUIStyle GUIStyleAchievementEarned, GUIStyle GUIStyleAchievementNotEarned)
     {
         if (Input.GetKey(KeyCode.A))
         {
         GUIStyle style = GUIStyleAchievementNotEarned;
         if (Earned)
         {
             style = GUIStyleAchievementEarned;
         }
 
         GUI.BeginGroup(position);
         GUI.Box(new Rect(0.0f, 0.0f, position.width, position.height), "");
 
         if (Earned)
         {
             GUI.Box(new Rect(0.0f, 0.0f, position.height, position.height), IconComplete);
         }
         else
         {
             GUI.Box(new Rect(0.0f, 0.0f, position.height, position.height), IconIncomplete);
         }
 
         GUI.Label(new Rect(80.0f, 5.0f, position.width - 80.0f - 50.0f, 25.0f), Name, style);
 
         if (Secret && !Earned)
         {
             GUI.Label(new Rect(80.0f, 25.0f, position.width - 80.0f, 25.0f), "Description Hidden!", style);
             GUI.Label(new Rect(position.width - 50.0f, 5.0f, 25.0f, 25.0f), "???", style);
             GUI.Label(new Rect(position.width - 250.0f, 50.0f, 250.0f, 25.0f), "Progress Hidden!", style);
         }
         else
         {
             GUI.Label(new Rect(80.0f, 25.0f, position.width - 80.0f, 25.0f), Description, style);
             GUI.Label(new Rect(position.width - 50.0f, 5.0f, 25.0f, 25.0f), RewardPoints.ToString(), style);
             GUI.Label(new Rect(position.width - 250.0f, 50.0f, 250.0f, 25.0f), "Progress: [" + currentProgress.ToString("0.#") + " out of " + TargetProgress.ToString("0.#") + "]", style);
         }
 
         GUI.EndGroup();
     }
     }
 }
 
 public class AchievementManager : MonoBehaviour
 {
     public Achievement[] Achievements;
     public AudioClip EarnedSound;
     public GUIStyle GUIStyleAchievementEarned;
     public GUIStyle GUIStyleAchievementNotEarned;
 
     private int currentRewardPoints = 0;
     private int potentialRewardPoints = 0;
     private Vector2 achievementScrollviewLocation = Vector2.zero;
 
     void Start()
     {
         ValidateAchievements();
         UpdateRewardPointTotals();
     }
     
     // Make sure the setup assumptions we have are met.
     private void ValidateAchievements()
     {
         ArrayList usedNames = new ArrayList();
         foreach (Achievement achievement in Achievements)
         {
             if (achievement.RewardPoints < 0)
             {
                 Debug.LogError("AchievementManager::ValidateAchievements() - Achievement with negative RewardPoints! " + achievement.Name + " gives " + achievement.RewardPoints + " points!");
             }
 
             if (usedNames.Contains(achievement.Name))
             {
                 Debug.LogError("AchievementManager::ValidateAchievements() - Duplicate achievement names! " + achievement.Name + " found more than once!");
             }
             usedNames.Add(achievement.Name);
         }
     }
 
     private Achievement GetAchievementByName(string achievementName)
     {
         return Achievements.FirstOrDefault(achievement => achievement.Name == achievementName);
     }
 
     private void UpdateRewardPointTotals()
     {
         currentRewardPoints = 0;
         potentialRewardPoints = 0;
 
         foreach (Achievement achievement in Achievements)
         {
             if (achievement.Earned)
             {
                 currentRewardPoints += achievement.RewardPoints;
             }
 
             potentialRewardPoints += achievement.RewardPoints;
         }
     }
 
     public void AchievementEarned()
     {
         UpdateRewardPointTotals();
         AudioSource.PlayClipAtPoint(EarnedSound, Camera.main.transform.position);        
     }
 
     public void AddProgressToAchievement(string achievementName, float progressAmount)
     {
         Achievement achievement = GetAchievementByName(achievementName);
         if (achievement == null)
         {
             Debug.LogWarning("AchievementManager::AddProgressToAchievement() - Trying to add progress to an achievemnet that doesn't exist: " + achievementName);
             return;
         }
 
         if (achievement.AddProgress(progressAmount))
         {
             AchievementEarned();
         }
     }
 
     public void SetProgressToAchievement(string achievementName, float newProgress)
     {
         Achievement achievement = GetAchievementByName(achievementName);
         if (achievement == null)
         {
             Debug.LogWarning("AchievementManager::SetProgressToAchievement() - Trying to add progress to an achievemnet that doesn't exist: " + achievementName);
             return;
         }
 
         if (achievement.SetProgress(newProgress))
         {
             AchievementEarned();
         }
     }
 
     void OnGUI()
     { 
         if (Input.GetKey(KeyCode.A))
         {
         float yValue = 5.0f;
         float achievementGUIWidth = 500.0f;
 
         GUI.Label(new Rect(200.0f, 5.0f, 200.0f, 25.0f), "CELE");
 
         // Setup a scrollview, and then fill it with each achievement in our list.
 
         achievementScrollviewLocation = GUI.BeginScrollView(new Rect(0.0f, 25.0f, achievementGUIWidth + 25.0f, 400.0f), achievementScrollviewLocation,
                                                             new Rect(0.0f, 0.0f, achievementGUIWidth, Achievements.Count() * 80.0f));
 
         foreach (Achievement achievement in Achievements)
         {
             Rect position = new Rect(5.0f, yValue, achievementGUIWidth, 75.0f);
             achievement.OnGUI(position, GUIStyleAchievementEarned, GUIStyleAchievementNotEarned);
             yValue += 80.0f;
         }
 
         GUI.EndScrollView();
 
         GUI.Label(new Rect(10.0f, 440.0f, 200.0f, 25.0f), "Reward Points: [" + currentRewardPoints + " out of " + potentialRewardPoints + "]");
         }
     }
 }

and a script, who add a progress if collide with mesh

 using System.Linq;
 using UnityEngine;
 using System.Collections;
 
 public class mesh : MonoBehaviour {
     void OnCollisionEnter(Collision collision) {
         foreach (ContactPoint contact in collision.contacts) {
            AchievementManager.AddProgressToAchievement("Ucieknij przed człowiekiem!", 5.0f);
         }
     }
 }

when i compile the script script called mesh, unity wrote in debug Assets/mesh.cs(8,31): error CS0120: An object reference is required to access non-static member `AchievementManager.AddProgressToAchievement(string, float)' Can anyone help me with this code?